From what I’ve been, the zero-knowledge proof system allows revealing attributes (which is a trivial ZKP), and to prove predicates such as >, <, = on fields in a credential. From what I understand, there is no theoretical limitation on the kinds of predicates for which a ZKP can be generated. Is this something Sovrin is working on? (Or does it already exist & am I missing information)

Indy-SDK currently supports predicates proofs for >=.
I personally have written code for implementing <=, <, >.
I need to finish testing it but plan to submit it as a PR soon. Just need to get more time to do this.